Apparatus for measuring a small gap using a Savart plate

ABSTRACT

An apparatus that can measure a space between a first surface and a second surface such as the air bearing between a slider and a disk. The apparatus may include a light source that can reflect a light beam from the slider and the disk. By way of example, the light beam can be reflected off of an Al2O3 cap of a slider. A birefringent element such as a Savart plate may split the reflected light beam into an ordinary beam and an extraordinary beam. The ordinary and extraordinary beams may combine to form an interference pattern that is detected by a photodetector. A controller receives data from the photodetector. The apparatus may have a mechanism which can vary a phase between the ordinary and extraordinary beams so that the controller can calculate a phase value phi. The controller then computes the space from the phase value phi. The variation in phase between the beams may be created by tilting the birefringent element, or moving the reflected light beam directed into the birefringent element. The apparatus can determine the space without performing a retract calibration routine that moves the slider as is required in the prior art.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical apparatus such as a flyingheight tester for determining a space between a first surface and asecond surface, such as the space between a slider and a transparentrotating disk.

2. Background Information

Hard disk drives contain magnetic transducers which write and readinformation onto a rotating magnetic disk(s). The transducers aretypically integrated into a slider that is assembled to a flexure arm.Some sliders contain a transducer to write information and a separatetransducer to read information. The read transducer may be constructedfrom a magneto-resistive (MR) material. The slider and arm are commonlyreferred to as a head gimbal assembly (HGA). Each HGA is attached to anactuator arm that can move the sliders across the surfaces of thedisk(s).

Each slider has an air bearing surface which cooperates with an air flowgenerated by the rotating disk(s) to create an air bearing between thedisk and the transducer. The air bearing prevents mechanical wearbetween the slider and the disk surface. It is desirable to minimize thelength of the space which separates the transducer and the disk tomaximize the magnetic coupling between the two components. Sliders aretherefore designed to create an optimal space between the transducer andthe disk.

It is desirable to measure the thickness of an air bearing created by aslider. The thickness, or “flying height,” is typically measured with anoptical system that places a slider adjacent to a rotating transparentdisk. A light beam is then directed through the transparent disk andreflected from the slider back to a photodetector. The detected light isused to compute the flying height of the slider. The current industrystandard is the Dynamic Flying Height Tester (DFHT) sold by PhaseMetrics, Inc. the assignee of the present application. The DFHT utilizesmultiple wavelength intensity based interferometry to determine theflying height. The operation of the DFHT is discussed in U.S. Pat. No.5,280,340 issued to Lacey, and U.S. Pat. No. 5,457,534 issued to Laceyet al.

The transducers may be fabricated by depositing thin films of materialonto a wafer substrate constructed from AlTiC. A final capping layer ofAl₂O₃ is deposited to protect the transducer. The wafer is then slicedinto individual slider elements such that the air bearing surface isprimarily AlTiC material. The AlTiC air bearing surfaces may be furthercovered with a protective coating of diamond-like carbon (DLC).

The sliders may have rails located along an air bearing surface whichassist in creating the air bearing. The transducers are located at thetrailing edge of the rails. The sliders typically “fly” at an obliqueangle relative to the disk so that the transducers are closer to thedisk surface than most of the surface of the rails.

Flying height testers are typically operated so that the light beam isreflected from the rails of the slider. Unfortunately this does notprovide an accurate measurement of the distance between the disk and thetransducer located at the trailing edge of the slider. As discussed inan article by Yufeng Li, ASME/STLE Joint Tribology Conference, SanFrancisco, Calif., Oct. 13-17, 1996, it is desirable to direct the lightbeam onto the Al₂O₃ cap of the transducer to evaluate the tribologicaland magnetic performance of the slider.

AlTiC is a granular material which may have varying grain sizesthroughout the air bearing surface. The non-uniform grain sizes can varythe optical properties at different locations of the air bearingsurface. To accurately determine the flying height, the DFHT must becalibrated to compensate for changes in optical properties. The DFHT iscalibrated by moving the slider away from the disk and recording data.

The DFHT has a loader which places the slider adjacent to the disk. Theloader pivots between a load position and an unload position. Thepivotal movement of the slider during the calibration routine introducesa rotation which may change the location on the slider from which thelight beam is being reflected. Unfortunately, the new location may havea different optical property that will degrade the accuracy of thecalibration routine and the measurements by the flying height tester.

This problem becomes particularly acute when trying to measure the Al₂O₃cap. Al₂O₃ has optical properties which are different than the opticalproperties of AlTiC. Therefore movement of the light beam from Al₂O₃ toAlTiC during the calibration routine may create an inaccuratecalibration and incorrect flying height measurements because of thedrastic change in optical properties. It would therefore be desirable toprovide a flying height tester which can accurately measure at the Al₂O₃cap of a hard disk drive slider.

The DFHT utilizes algorithms that are dependent upon the complex indexof refraction (n and k), which is an optical property of the slidermaterial. The complex index of refraction for each batch of sliders istypically measured with an ellipsometer and stored in the DFHT for usein computing the flying height. Having to separately determine thecomplex index of refraction is time consuming and thus increases thecost of testing the sliders and mass producing hard disk drives.

There has been marketed a flying height tester by Zygo Corp. under thetrademark PEGASUS 2000 FHT which determines both the complex index ofrefraction and the flying height of a slider. The Zygo machine isfurther explained in U.S. Pat. No. 5,557,399 issued to DeGroot. The Zygomachine utilizes a polarized light beam that is reflected from the diskand the slider at an oblique angle. The approach implemented in the Zygomachine has the following problems.

It is difficult to analyze a polarized beam that is reflected from aspinning glass disk. The centrifugal forces generate stresses which makethe glass birefringent. Birefringence is a characteristic of somematerials that can be described as if the material had different indicesof refraction depending on the polarization state of the light. In thecontext of Polarization-based FHTs, such as the Zygo machine, it meansthat the polarization states of both the probing beam and of thereflected beam that carries the information on flying height are changedas they travel through the glass disk. These effects are of suchmagnitude in regular flying height testing conditions that they renderthe measurements meaningless if they are not corrected. A complexcorrection method for the Zygo machine is described in U.S. Pat. No.5,644,562, issued to De Groot.

The requirement for oblique incidence poses difficulties when it comesto accurately positioning the measurement spot on the surface of theslider. The Zygo machine requires a compromise between eitherdetermining the spot position from a low quality, high angleperspective, or necessitates an extra normal incidence view channel thatmust be precisely registered to the oblique measurement channel. Thisregistration is susceptible to mechanical drift and in addition needs tobe redone every time the transparent disk is changed, a task that isregularly performed in mass testing environments. Additionally, eventhough the three unknowns of flying height testing, flying height, andthe components of the complex index of refraction n and k, areindependent variables, they are closely coupled in theellipsometric-type model that is used in the Zygo machine to interpretthe data, resulting in solutions that are naturally unstable and tend toproduce correlated fluctuations in the flying height and complex indexof refraction.

Another problem in measuring flying heights is the granular nature ofthe AlTiC slider material. When a beam of light impinges on the slidersurface, it is not cleanly reflected in a specular fashion. Instead, alarge fraction of the light is scattered off in a diffuse manner, andeven the portion of the light that reflects specularly getssignificantly depolarized. It is extremely difficult to properly accountfor these effects in the theoretical model that is used for analyzingthe data, since all the common approximations break down in the regimein which the grain size is of the order of the wavelength of the light.This results in having to introduce other ad hoc correction factors,such as what percentage of the light actually makes it back to thedetector after reflection ,and what fraction of the detected lightretains the polarization information required to determine the flyingheight.

The DLC coating of the air bearing surface introduces yet anotherproblem. An error in the flying height measurement is introduced in theZygo machine since the model interprets the various reflections from thedifferent interfaces within the material as a single reflection, causedby an average material, located at a certain depth beneath the actualsurface. This effect appears also at normal incidence, but forDLC-coated AlTiC its magnitude is close to a factor of 5 times smallerat normal incidence that at angles between 45 and 60 degrees used in theZygo machine. The effect is important if the indices of refraction ofthe substrate (AlTiC in this case), and the coating (DLC) are verydifferent.

It would be desirable to have a flying height tester which canaccurately measure the distance between a disk and a slider whileovercoming the noted problems in the prior art.

SUMMARY OF THE INVENTION

One embodiment of the present invention is an apparatus that can measurea space between a first surface and a second surface. The apparatus mayinclude a light source that can reflect a light beam from the first andsecond surfaces. A birefringent element may split the reflected lightbeam into an ordinary beam and an extraordinary beam. An interferencebetween the ordinary and extraordinary beams is detected by aphotodetector. A controller receives data from the photodetector. Theapparatus may have a mechanism which can create relative phase shiftsbetween the ordinary and extraordinary beams so that the controller cancalculate a phase value φ. The controller then computes the space fromthe phase value φ.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an embodiment of an apparatus of the presentinvention;

FIG. 2 is a bottom view of a double image on a photodetector;

FIG. 3 is a graph showing a correlation between a phase value and aflying height.

DETAILED DESCRIPTION

One embodiment of the present invention is an apparatus that can measurea space between a first surface and a second surface such as the airbearing between a slider and a disk. The apparatus may include a lightsource that can reflect a light beam from the slider and the disk. Byway of example, the light beam can be reflected off of an Al₂O₃ cap of aslider. A birefringent element such as a Savart plate may split thereflected light beam into an ordinary beam and an extraordinary beam.The ordinary and extraordinary beams may combine to form an interferencepattern that is detected by a photodetector. A controller receives datafrom the photodetector. The apparatus may have a mechanism which cancreate relative movement between the ordinary and extraordinary beams sothat the controller can calculate a phase value φ. The controller thencomputes the space from the phase value φ. The relative movement of thebeams may be created by tilting the birefringent element, or moving thereflected light beam directed into the birefringent element. Theapparatus can determine the space without performing a retractcalibration routine that moves the slider as is required in the priorart.

Referring to the drawings more particularly by reference numbers, FIG. 1shows an embodiment of an apparatus 10 of the present invention. Theapparatus 10 may be a flying height tester which measures the space 12between a slider 14 and a rotating transparent disk 16. Generally, thedisk 16 may be described as a first surface and the slider 14 may bedescribed as a second surface. The slider 14 may have a transducer 18that is covered with an Al₂O₃ cap 19. The remaining portion of theslider 14 may be constructed from AlTiC material.

The slider 14 is typically integrated into a head gimbal assembly (HGA)20 which is eventually assembled into a hard disk drive. The apparatus10 can be used to measure a space such as an air bearing “flying height”that is formed by an air bearing surface 22 of the slider 14 and an airflow created by the rotating disk 16. The apparatus 10 may have a loader24 that can retain the HGA 20 and place the slider 14 adjacent to thedisk 16. An operator of the apparatus typically replaces the HGA 20 withanother part after the flying height has been measured. The transparentdisk 16 is rotated by a motor 26 that is integrated into a spindlestand.

The apparatus 10 may include a light source 28 that generates a lightbeam 30 which is focused onto an aperture 32 by a lens 34. The lightbeam 30 passes through a slit 36 of the aperture 32. The lens 40 focusesthe light beam in a manner to form an image of the slit 36 at or closeto the back focal plane 42 of lens 44, after reflecting off abeamsplitter 38. Lens 44 directs a substantially partially collimatedlight beam onto the disk 16 and slider 14. By partially collimated wemean a light beam that includes rays that are restricted to travel in aplane of collimation. The plane of collimation may include the opticalaxis and a direction substantially parallel to the trailing edge of theslider 14.

The partially collimated light is reflected from the disk 16 and theslider 14. The light is reflected through the beamsplitter 38 and lens46. The lens 46 has a focal point located at the back focal plane 42 oflens 44.

The lens 46 directs the light through a first polarizer 48, abirefringent element 50 and a second polarizer 52. The polarizer 48 maybe either parallel or perpendicular to the plane of collimation. Thelight beam that emerges from the polarizer 48 may thus have componentslinearly polarized at plus and minus 45° from the plane of collimation,although it is to be understood that the beam may have componentspolarized at angles greater or less than 45°.

The birefringent element 50 splits the polarized light beam into anordinary beam(s) 54 and an extraordinary beam(s) 56 which have a phaserelative to each other. In essence the element 50 creates a double imageof the disk 16 and the slider 14. The birefringent element 50 may be aSavart plate which contains two substantially equal pieces ofbirefringent material such as calcite or quartz that are cut at acrystalline direction and attached to each other. A Savart platesuitable for the present invention can be purchased from Karl LambrechtCorp. of Chicago Ill.

Savart plates provide minimal retardation so that the ordinary 54 andextraordinary 56 beams can interfere to form an interference pattern.Although a Savart plate is shown and described, it is to be understoodthat other birefringent elements may be used in the present invention.For example, a single birefringent element may be used, particularly ifthe light source is a laser or laser diode.

In a technical sense, the outcoming beams of a Savart plate cannot becalled “ordinary” and “extraordinary.” Since the Savart plate consistsof two pieces of birefringent material, the initial pair of ordinarybeam and extraordinary beam switch places when they enter the secondpiece of material, respectively becoming extraordinary and ordinary.This application will, nevertheless, use the names ordinary andextraordinary in an ample way to include cases involving a single pieceof birefringent material.

The ordinary 54 and extraordinary 56 beams are made to interfere witheach other by polarizer 52, filtered by a color filter 58 and detectedby a photodetector 60. The photodetector 60 may be a charge coupleddevice (CCD) camera which contains a plurality of individual pixelsarranged in a two dimension array. The photodetector 60 detects theinterference pattern of the ordinary 54 and extraordinary 56 beams.

FIG. 2 shows the double image 70 and 72 on the photodetector. Thedistance between the images 70 and 72 is referred to as a shear whichhas a corresponding shear direction. The shear direction is dictated bythe orientation of the Savart plate. The orientation of the Savart platealso determines the orientation of the polarizers and slit.

The photodetector may have a number of pixels 74, 76, 78, 80 and 82, orgroups of pixels, which sense different modulations of the interferinglight. For example, pixel 74 will sense light only reflected from thedisk. Pixel 76 will sense light that is reflected from the slider/diskinterface at the location of the Al₂O₃ cap as provided in image 70, andlight reflected from just the disk as provided by image 72. Pixel 78 maysense light that is reflected from the slider/disk interface at thelocation of the AlTiC material as provided by image 70, and lightreflected from just the disk as provided by image 72. Pixel 80 may senselight that is reflected from the slider/disk interface at the locationof the Al₂O₃ cap as provided by image 72, and light reflected from theslider/disk interface at the location of the AlTiC material as providedby image 70. Finally, pixel 82 may sense light that is reflected fromthe slider/disk interface at a location of the AlTiC material asprovided by images 70 and 72.

It being understood that the light reflected from the slider/diskinterface contains phase information that can be related to the flyingheight in accordance with the following equations. $\begin{matrix}{\varphi = {{\arctan \left\lbrack {R_{s}R_{g}\quad {{\sin (\delta)}/\left\{ {1 - {R_{s}R_{g}\quad {\cos (\delta)}}} \right\}}} \right\rbrack} - {\arctan \left\lbrack {R_{s}\quad {{\sin (\delta)}/\left\{ {R_{g} - {R_{s}\quad {\cos (\delta)}}} \right\}}} \right\rbrack}}} & (1) \\{\delta = {4\quad \pi \quad {h/\lambda}}} & (2)\end{matrix}$

where;

φ = a measured phase value;

h = the flying height;

R_(g) = the reflectance of the glass-air interface, which can bepredetermined;

R_(s)=the reflectance of the air-slider interface which is defined bythe equation (n−1)/(n+1) and can be computed or predetermined, n beingthe real index of refraction of the slider;

λ=the wavelength of the reflected light.

Referring to FIG. 1, the photodetector 60 can be coupled to a controller84. The photodetector 60 may provide electrical data signals which are afunction of the measured intensity of the reflected light. Thecontroller 84 may be a computer which has a microprocessor, memory,interface circuits, etc., which can receive the data signals from thedetector 60 and perform software algorithms to determine differentvariables including the phase value φ and flying height h.

When measuring the flying height between the Al₂O₃ cap and the disk,data from pixel 76 shown in FIG. 2, can be used to calculate the phasevalue φ and the flying height h. The controller 84 can receive the datafrom the detector and determine the phase value φ. The controller 84 canthen utilize equations (1) and (2) to compute the flying height h fromthe measured phase value φ. A graph showing the correlation between thephase value φ and the flying height h is shown in FIG. 3.

The phase value φ can be measured in accordance with a technique that isdiscussed in “Temporal Phase Measurement Methods”, by K. Creath, inInterferogram Analysis: digital fringe pattern measurement techniques,Ed. D. W. Robinson and G. T. Reid, IOP Publishing Ltd. London (1993).This technique requires a recordation of a succession of slider/diskimages at regular changes in phase retardation.

The relative phase retardation of the ordinary and extraordinary beamscan be varied by tilting the incoming beam relative to the Savart plate.The relevant tilt can be accomplished by moving the beam in a planedefined by the optical axis and the shear direction of the system. Inone embodiment, the light beam is tilted by moving the aperture 32 witha mechanism 86 shown in FIG. 1. Alternatively, the image of the slit canbe moved by a glass plate (not shown) that is located between the slit36 and lens 40 and is tilted by a mechanism. As another embodiment, theSavart plate can be tilted by a mechanism.

A minimum of three images are required to obtain the phase value φ,although it is to be understood that a larger number of frames can betaken. In one embodiment, seven frames are taken to determine the phasevalue φ.

The phase measured at pixel 76 will differ from the true value of φ by aconstant that will depend on a number of factors such as the particulararrangement of the optical components, and the starting point of thephase shifting used to determine the phases. The constant can bedetermined by performing a phase measurement at the same pixel of thedetector when the slider is not adjacent to the disk. This can be doneeither by mechanically displacing the slider out of the view of thepixel, or measuring the light intensity at the pixel before the slideris loaded onto the disk. The second approach is preferred due toperformance considerations.

The phase value constant is affected by thermal drifts in the apparatus,so it tends to change over time, and needs to be updated every fewminutes to insure accurate measurements. In the preferred embodiment inwhich detector 60 is an image detector, a time evolution at pictureelement 74 can be used for correcting the drift, enabling accuratemeasurements to be taken for periods of hours without constantlyperforming the phase value constant calibration. Also, for a perfectsystem in which the additive phase is constant throughout the entirefield of view, the phase measured at picture element 76 can bereferenced to the phase measured at picture element 74. In oneembodiment, the expression

φ_(76c)=φ₇₆−φ_(76G)−(φ₇₄−φ_(74G))  (3)

can be used to correct for both spatial and temporal variations of thephase. φ₇₆ and φ₇₄ are the phases respectively measured at pixels 76 and74 when the slider is loaded adjacent to the disk, while φ_(76G) andφ_(74G) are the phase values measured at the same detector locationswhen the slider is not adjacent to the disk. φ_(76c) is the correctedphase value to be used in the flying height determination of Equations(1) and (2).

For high speed dynamic measurements, the detector 60 may be a line scancamera or a fast photodetector containing a few detector elements. Aninitial measurement is performed similarly to the area detectormeasurements previously described. Then the phase shift is preferablyadjusted to yield an intensity reading on the detector close to thecenter of the range of intensities to get high sensitivity, andintensities are then recorded at high speed to obtain the desireddynamic information. Alternatively, the fast streams of data aregathered at each different phase shift. The average spacing isdetermined with high accuracy from the averages of the data streams, andthe data stream or streams with the highest sensitivity are then used toevaluate the dynamic behavior.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

Although measurements of flying height at the Al₂O₃ cap of the slider,where the index of refraction is real, has been described, it is to beunderstood that the present invention allows measurements to beperformed on the AlTiC portion of the slider as well. This would requirethe expression for R_(s) to be rewritten for a material with a complexindex of refraction, and the introduction of a correction for the phasesift on reflection, as taught in U.S. Pat. Nos. 5,280,340 and 5,457,534,which are hereby incorporated by reference.

What is claimed is:
 1. An apparatus that can measure a space between afirst surface and a second surface, comprising: a light source thatreflects an unpolarized light beam from the first and second surfaces ata normal incidence; a birefringent element that splits the reflectedlight beam into an ordinary beam and an extraordinary beam which have aphase relative to each other; a mechanism which can vary the phasebetween the ordinary and the extraordinary beams; a photodetector whichdetects the ordinary and the extraordinary beams; a controller that iscoupled to said photodetector and which can compute the space betweenthe first and second surface from the detected ordinary andextraordinary beams.
 2. The apparatus as recited in claim 1, whereinsaid controller initially computes a phase value which is used tocompute the space.
 3. The apparatus as recited in claim 1, wherein saidbirefringent element is a Savart plate.
 4. The apparatus as recited inclaim 1, further comprising a first polarizer located between saidbirefringent element and the first and second surfaces.
 5. The apparatusas recited in claim 1, further comprising a polarizer located betweensaid birefringent element and said photodetector.
 6. The apparatus asrecited in claim 1, wherein said photodetector detects an interferencepattern of the ordinary and extraordinary beams.
 7. The apparatus asrecited in claim 2, further comprising an aperture located between saidlight source and the first and second surfaces.
 8. The apparatus asrecited in claim 7, wherein said mechanism displaces the image of saidaperture.
 9. The apparatus as recited in claim 2, wherein said mechanismmoves said birefringent element.
 10. A flying height tester that canmeasure a space between a slider and a disk, comprising: a transparentdisk; a motor that rotates said transparent disk; a loader which canplace the slider adjacent to said transparent disk so that the sliderand said transparent disk are separated by a space; a light source thatreflects an unpolarized light beam from the slider and said transparentdisk at a normal incidence; a birefringent element that splits thereflected light beam into a ordinary beam and an extraordinary beam; amechanism which can create a relative movement between the ordinary andthe extraordinary beams; a photodetector which detects the ordinary andextraordinary beams; a controller that is coupled to said photodetectorand which can compute the space between the slider and said transparentdisk from the detected ordinary and extraordinary beams.
 11. Theapparatus as recited in claim 10, wherein said controller initiallycomputes a phase value which is used to compute the space.
 12. Theapparatus as recited in claim 10, wherein said birefringent element is aSavart plate.
 13. The apparatus as recited in claim 10, furthercomprising a first polarizer located between said birefringent elementand said transparent disk.
 14. The apparatus as recited in claim 10,further comprising a polarizer located between said birefringent elementand said photodetector.
 15. The apparatus as recited in claim 10,wherein said photodetector detects an interference pattern of theordinary and extraordinary beams.
 16. The apparatus as recited in claim11, further comprising an aperture located between said light source andsaid transparent disk.
 17. The apparatus as recited in claim 16, whereinsaid mechanism displaces said aperture.
 18. The apparatus as recited inclaim 11, wherein said mechanism moves said birefringent element.
 19. Amethod for determining a space between a first surface and a secondsurface, comprising; a) reflecting an unpolarized light beam from thefirst and second surfaces at a normal incidence; b) splitting thereflected light into an ordinary beam and an extraordinary beam; c)retarding the ordinary and extraordinary beams relative to each other;d) detecting the ordinary and extraordinary beams; and, e) computing thespace from the detected ordinary and extraordinary beams.
 20. The methodas recited in claim 19, wherein the reflected light is split with abirefringent element.